Impact of particles on photolysis rates and air quality

Photolysis rates in the atmosphere are greatly affected by the presence of clouds and aerosols. In regional Chemistry Transport Model (CTM) used for air quality modelling, the impact of particles on photolysis rates are generally not taken into account. In this study we coupled a regional CTM (Polair-3D from the Polyphemus platform) to a photolysis scheme model (Fast-JX). The impact of particles on photolysis rates and on gas and aerosol concentrations is evaluated in July 2001 over Europe. Taking into account particles when computing photolysis rates reduce photolysis rates all over the troposphere with maximum reduction at the ground (around 13%). This leads to a reduction of ground O₃ concentrations by a few percent but O₃ peaks are much more reduced, especially locally, close to precursors (NOx) emission sites. It was found that simulated O₃ exceedances of the information threshold value (hourly O₃ concentration > 180 μg.m^-3) and to the alert threshold (hourly O₃ concentration > 240 μg.m^-3) are reduced by about 50 % when including aerosol feedbacks on photolysis rates. Concentrations of oxidant species such as OH are also greatly affected through all the troposphere, leading to changes in concentrations of VOC, but also of secondary aerosols especially at the ground. For example, monthly tropospheric isoprene concentrations increase by 8 % over Europe. Secondary aerosols such as sulphate or secondary organic aerosol are also affected but to a lesser extend. Aerosol having the strongest impact is dust due to its strong loading over south Europe. This study shows the importance of taking into account the particles effect on photolysis for both gas and aerosol ground concentrations and therefore for air quality monitoring.